The invention relates to a process for the powder metallurgical production of objects from tool steel with improved homogeneity, greater purity, and improved properties.
The invention further relates to a tool steel object with an improved property profile.
Tool steels with a high concentration of carbon and high contents of carbide-forming elements are used for cutting parts and components with a high degree of wear resistance. Because inhomogeneities as well as coarse primary and eutectic carbides are formed during solidification of such alloys in casting molds, which cause production problems and poor mechanical characteristics of the tools or components produced therefrom, a powder metallurgical production of such parts is advantageous.
A powder metallurgical production essentially includes atomizing of a tool steel melt into metal powder, collection and compaction of the metal powder in a container or capsule, respectively, sealing the capsule, and heating and hot isostatic pressing of the powder in the capsule into a dense, homogeneous material.
When the melt is being atomized which, according to the prior art, advantageously is done using nitrogen, small metal droplets with a high ratio of surface to volume are formed in the gas stream, which causes a high cooling and solidification speed of the liquid metal and, therefore, small carbide particles in the powder grains. As mentioned previously, the powder, which is usually compacted in the capsule by means of tapping, is formed in the capsule by means of hot isostatic pressing at temperatures usually greater than 1080xc2x0 C. at a pressure greater than 85 MPa into a completely dense metal body. This AS HIPed metal body, which may be subjected to another hot forming process, at a high carbon content has an advantageously low carbide size of an average of 1-3 um and good mechanical properties as compared to production using melt metallurgy.
While objects produced from tool steel using powder metallurgy do have an advantageous structure with finely distributed carbide phases, the high quality potential of PM materials that can be achieved cannot be realized due to an incomplete material isotropy and a poor degree of purity.
Here, the invention is intended to provide a remedy and has the object of eliminating the lack of quality of the objects produced from PM tool steel according to the prior art and providing a process of the type mentioned at the outset with which an isostatically pressed metal body that has maximal material isotropy and a minimal content of oxide inclusions may be produced.
A further goal of the invention is a tool steel object with improved processing and use properties at an increased useful service life.
This goal is attained in a process of the above type in that a melt is placed into a metallurgical vessel and is conditioned therein, this is an improvement in the degree of oxide purity of the same and an adjustment of the temperature to a value above the formation of primary precipitations in the alloy, whereupon, with the temperature being kept essentially constant, a powder that has an average grain diameter of 50 to 70 xcexcm is produced from this melt by means of atomizing with nitrogen, is disintegrated in the nitrogen stream, and, while maintaining the nitrogen atmosphere, the powder with a maximum grain diameter of 500 xcexcm is classified, collected, mixed, and placed into a container with a diameter or thickness greater than 300 mm and a length greater than 1000 mm, compacted in this container or capsule by means of mechanical strokes, and the container is sealed in a gas-tight manner, whereupon, in a hot isostatic pressing cycle therefor, the parameters are set in such a way that the temperature and pressure are increased in the heating process, wherein a pressure of 1 to 40 MPa from all sides is exerted in the powder body of the container or capsule, respectively, and subsequently an isostatic pressing operation occurs at a temperature of at least 1100xc2x0 C., but a maximum of 1180xc2x0 C., at an isostatic pressure of at least 90 MPa for a period of time of at least three hours and the HIP pressed body is then cooled and, optionally, this pressed body is subsequently hot formed and, in this manner, a material with a K0 value according to DIN 50 602 of essentially a maximum of 3 is produced.
The advantages achieved with the process according to the invention are essentially based on the fact that, synergetically by means of metallurgical working of a melt placed in a metallurgical vessel, initially the degree of oxidic purity of the melt is decisively improved and its temperature is homogeneously adjusted to an advantageous overheating value, whereupon an atomization of the liquid metal is carried out in such a way that the average grain diameter is 50 to 70 xcexcm. In this manner, it is achieved that, on the one hand, the oxygen content in the powder is surprisingly low and, on the other hand, the fine-grained portion is also significantly increased with respect to achieving a high tapping and vibration density in the capsule. If, as provided according to the invention, the metal powder is classified, collected, placed in a container, compressed in this container, and the container is sealed, all while maintaining the nitrogen atmosphere, no oxidation or physisorption of oxygen on the powder grain surface can occur.
A distribution according to the invention of the grain diameter with an average value in the range of 50 to 70 xcexcm allows an unexpectedly high powder density to be attained in the capsule such that, on the one hand, the degree of shrinkage thereof in hot isostatic pressing is low and, on the other hand, a substantially complete isotropy of the pressed dense metal body is present. These advantages are also achieved with container or capsule sizes with a diameter or a thickness greater than 300 mm and a length greater than 1000 mm.
The parameters for the hot isostatic pressing cycle include a heating of the powder in the capsule at a substantially simultaneous increase of temperature and pressure, whereby, as has been shown, an increase in the material density and homogeneity is achieved, already in this phase. The subsequent pressing operation is carried out in the temperature range of 1100xc2x0 C. to 1180xc2x0 C. at a pressure of 90 MPa and more with a duration of at least three hours, followed by a slow cooling of the pressed body. Lower pressing temperatures than 1100xc2x0 C. and pressures less than 90 MPa as well as lower pressing times than three hours can cause porosity in the material.
After HIPing, the pressed body has a completely dense material structure and can therefore by processed into a tool in this state or after another hot forming operation.
For the high quality of the tool steel object produced with powder metallurgy using the process according to the invention, its lower amount of inclusions as well as its low inclusion size are characteristic. The high oxidic degree of purity that is documented by a K0 value according to DIN 50 602 of essentially not higher than 3 not only leads to markedly improved mechanical characteristics of the material in all stress directions, in particular at high application temperatures, but also improves its use properties, preferably the edge-holding property of fine cutting tools, to a large degree.